PARIS — People who die after getting swine flu are up to 100 timesmore likely, compared to seasonal flu, to have been killed by thevirus itself rather than secondary causes, a top French researchersaid Monday.

It could also provide important clues as to the new swine flu'spotential virulence, said author Antoine Flahault, a leadingepidemiologist and director of France's School for Advanced Studies inPublic Health.

"The direct lethality due to viral pneumonia probably gives the bestestimate of an influenza strain's virulence, since it may vary fromstrain to strain and is not due to a country's level of healthdevelopment," he said.

With regular seasonal flu, which claims up to 500,000 lives each yearworldwide, most deaths are attributed either to secondary bacterialinfections such as pneumonia, or pre-existing chronic conditions thatboost vulnerability.

Only about one-in-a-million infections result in death due to a rarecondition known as acute respiratory disease syndrome (ARDS).

"ARDS is frighteningly lethal -- it is like drowning," Flahault toldAFP by phone. The condition requires intensive-care treatment for anaverage of thee weeks. "Statistically, only one in two patientssurvive."

The risk factors for ADRS are poorly understood, though recent studiesin the United States suggest that, among people infected with swineflu, pregnant women and the very obese are particularly at risk.

Both seasonal influenza and the new A(H1N1) virus that has swept theglobe since May appear to have roughly the same mortality rate of one-to-five per 1,000 infections, though figures for the swine flu remainvery sketchy.

Experts also caution that the pandemic flu could become more lethal asit continues to mutate.

But preliminary analysis of infection and mortality statistics fromthe French territory of New Caledonia and the Indian Ocean islandnation of Mauritius suggest that the new A(H1N1) virus directly causedan ARDS fatality for every 10,000 cases, Flahault told AFP.

In New Caledonia, local health authorities have reported 30,000infections and two deaths attributed directly to the pandemic virus.

Flahault said that some 70,000 persons have been infected inMauritius, with seven reported deaths from ARDS, five of themconfirmed.

"These surveillance data allow for a first estimate of directlethality due to H1N1 of one-per-10,000 infections, about 100 timesmore than regular seasonal flu," he said.

Flahault cautioned that these are only "preliminary and roughestimates," and acknowledged that his findings -- based on a limitednumber of cases on two isolated islands -- may not extend to muchlarger, continental countries.

"But we have a terrible lack of data," he said.

"It may be useful to deliver such estimates ... as early as possibleso health authorities can check availability of intensive care unitsand artificial ventilation devices in case of a wave of similarvirulence this autumn in the northern hemisphere," he said.